The present invention relates to an improved method for protecting cast austenitic stainless steel tubes against carburization when such tubes are employed in high temperature carburizing environments, such as in the steam cracking of hydrocarbons.
A variety of heat-resistant alloy steels have been developed for use in pyrolysis furnaces for the thermal decomposition of organic compounds, such as the steam cracking of hydrocarbons. Generally, the pyrolysis furnace contains a series of heat-resistant alloy steel tubes in which the reaction occurs. It will be noted that the term "tube" as used herein also includes fittings, pipes and other parts used to contain carburizing materials.
While it is well known that alloy steels containing various amounts of nickel and chromium are useful in constructing heat-resistant pyrolysis tubes having relatively long performance lives, premature tube failure continues to be the limiting problem. A major cause of such failure is carburization of the tubes brought about by the extremely high temperatures and carburizing atmospheres encountered. Carburization of such tubes, which is the diffusion of carbon into the alloy steel causing the formation of additional carbides, brings about the embrittling thereof. Once the steel has become embrittled, it is susceptible to creep rupture failure, and/or brittle fracture due to thermal stress. Carburization often occurs at localized spots in the tubes, and of course when this has proceeded to the point of failure or potential failure, even at only one spot, the tubing must be replaced.
Although progress has been made in improving the carburization resistance of such tubes by conventionally machining the inner surfaces of the tubes prior to use, it has been found that such machining only improves the carburization resistance to a very limited degree.